This invention relates to oxygen-separating porous membranes to be used in oxygen-enriching processes, typically for combustion gas production, for medical treatment. More particularly, the invention concerns porous membranes which contain, as dispersed in the pores, a metal complex capable of adsorbing and desorbing oxygen rapidly and reversibly.
Oxygen is one of the chemicals most widely used on industrial scales, specifically in the manufacture of iron, steel, and other metals and glass, in chemical oxidation and combustion, and in wastewater disposal. In the field of medical care too, it has very wide applications including the therapy for lung disease patients by means of oxygen inhalation. For these reasons the development of processes for concentrating oxygen out of air is an important problem with far-reaching effects on various sectors of industry. While dominant industrial processes for atmospheric oxygen concentration today are low-temperature and adsorption techniques, membrane separation is considered promising from the energy-saving viewpoint.
Success of membrane separation depends primarily on the discovery of a membrane material that would permit selective and efficient oxygen permeation relative to nitrogen from air. Currently available membranes capable of permeating and concentrating atmospheric oxygen (known as oxygen-enriching membranes) are those of silicone, silicone polycarbonate, and the like. Some of them are in practical service. They do not have high oxygen-permeation selectivity (oxygen-permeability coefficient/nitrogen-permeability coefficient, or ratio .alpha.), the value being approximately 2, and yet exhibit high permeability coefficient (10.sup.-8 [cm.sup.3 .multidot.(STP).multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg]). With this feature the membranes are incorporated in modules, multistage processes, and other systems to obtain oxygen-enriched air, with oxygen concentrations of approximately 30%.
Gas separation by the use of microporous membranes ranging in pore size from several ten to several hundred angstroms is also extensively performed. Gas permeation through a porous mass is dictated by the ratio of the distance over which the particular gas particles impinge upon one another, or the mean free path, .lambda. to the pore diameter r, (r/.lambda.). When the pore diameter is small, being r/.lambda.&lt;1, the mutual impingement of the gas particles is ignored. The permeation conforms to the Knudsen flow in which it is inversely proportional to the square root of the molecular weight of the gas. Gas permeation based on this permeation mechanism attains a strikingly high permeability coefficient. Nevertheless, the process is unsuitable for the oxygen separation by permeation from air, because, when separating gases of dissimilar molecular diameter, such as oxygen and nitrogen, the selectivity becomes less than 1.
It has been reported that generally gas molecules once adsorbed by the pore surface of a porous membrane will diffuse over the adsorption layer for permeation, resulting in a substantial increase in permeability. However, the phenomenon is limited to operations handling lower hydrocarbons, carbonic gas, and other gases of relatively high boiling points. The phenomenon also is observed only when membranes with pore diameters from about 30 to about 300 .ANG. are used. Oxygen permeation from air has not in the least been known in the art.
In order to obtain highly oxygen-rich air useful for industrial and medical applications by a single permeable-membrane pass, it is essential that the separating membrane have a high oxygen-permeability coefficient, of the order of 10.sup.-8, and an .alpha. of at least 5.
Polymeric membranes of silicone and the like exhibit oxygen-permeability coefficients as high as about 10.sup.-8, but their oxygen selectivities are low. The porous membranes that rely upon the Knudsen flow for gas permeation are incapable of separating oxygen and nitrogen, although they show greater permeability than polymeric membranes.
We have hitherto continued the synthesis of metal complexes capable of rapid, reversible adsorption and desorption of oxygen molecules. As a result, we clarified essential requirements of the metal complexes that can adsorb and desorb oxygen molecules selectively, rapidly, and reversibly, even in a solid-phase polymer. We successfully synthesized the novel complexes and taught their use for oxygen-separating membranes (Japanese Patent Application Public Disclosure No. 171730/1987). However, polymeric membranes incorporating such complexes, when used in air permeation, did not always achieve the object satisfactorily. Although the .alpha. value exceeded the target value of 5, the permeability coefficient was only 10.sup.-9. For the treatment of a sufficiently large volume of air for oxygen enrichment, an additional step, for example, of providing an extra thin film, was required.